1. Field of the Invention
The present invention relates to a driving method and a driving device for writing an image in a stacked light modulating device, which contains plural selective reflection layer containing a liquid crystal and displays and records an image of two or more colors.
2. Description of the Related Art
There is increasing expectation of a rewritable marking technique as an alternative to paper as a hardcopy technique due to such reasons as protection of the global environment including forest resources and space saving.
A reflective liquid crystal display device is receiving attention as a display device for a compact information processing equipment and a personal digital assistant since it requires no dedicated light source like aback light, consumes a small amount of electric power, and can be constituted to a thin and small size.
Various techniques using a reflective liquid crystal display device have been proposed for the rewritable marking technique. Among these, the image writing and displaying method, which has been proposed by the inventors, by a threshold value shifting method using a stacked light modulating device provides significant advantages since two display layers can be independently controlled with one driving signal, which simplifies the structure of the medium and reduces the production cost (as described in JP-A-10-177191 and JP-A-2000-111942). Accordingly, the threshold value shifting method provides such a significant advantage that an image of two or more colors including a full-color image can be can be displayed and recorded with a simple manner at low cost.
The principle of the image writing and recording method using the threshold value shifting method will be described below.
FIG. 13 is a schematic cross sectional view showing a state of image writing by the threshold value shifting method. A stacked light modulating device 101 mainly has two display layers 107a and 107b each containing a cholesteric liquid crystal stacked between a pair of transparent electrodes 105 and 106.
A cholesteric liquid crystal having a positive dielectric anisotropy shows the following three states. That is, in a planar texture, the helical axis is perpendicular to the cell surface as shown in FIG. 14A, and the incident light is subjected to the aforementioned selective reflection phenomenon. In a focal conic texture, the helical axis is substantially in parallel to the cell surface as shown in FIG. 14B, and the incident light is transmitted with slightly forward scattering. In a homeotropic texture, the helical structure is unraveled to direct the liquid crystal director to the electric field direction as shown in FIG. 14C, and the incident light is substantially completely transmitted.
Among the three states, the planar texture and the focal conic texture can be bistably present under no electric field. Therefore, the texture state of a cholesteric liquid crystal is not determined unconditionally without the intensity of the electric field applied to the liquid crystal layer, and in the case where a planar texture appears as the initial state, the texture state is changed sequentially to a planar texture, a focal conic texture and a homeotropic texture in this order with increase of the intensity of the electric field, and in the case where a focal conic texture appears as the initial state, the texture state is changed sequentially to a focal conic texture and a homeotropic texture in this order with increase of the intensity of the electric field.
In the case where the intensity of the electric field applied to the liquid crystal layer is decreased suddenly to zero, the planar texture and the focal conic texture maintain the states as they are, and the homeotropic texture is changed to a planar texture.
Therefore, the cholesteric liquid crystal layer immediately after applied with a pulse signal shows a switching behavior as shown in FIG. 15. That is, when the voltage of the pulse signal applied is Vfh, 90 or higher, a selective reflection state where a homeotropic texture is changed to a planar texture appears. When the voltage is from Vpf, 10 to Vfh, 10, a transmission state with a focal conic texture appears. When the voltage is Vpf, 90 or lower, the state before applying the pulse signal is continued, i.e., a selective reflection state with a planar texture or a transmission state with a focal conic texture appears.
In FIG. 15, the ordinate shows the normalized reflectance, which is obtained by normalizing the reflectance with the maximum reflectance as 100 and the minimum reflectance as 0. A transition state appears among the planar texture, the focal conic texture and the homeotropic texture, and therefore, it is determined that the case where the normalized reflectance is 90 or more is designated as the selective reflection state, the case where the normalized reflectance is 10 or less is designated as the transmission state, the threshold voltage of texture transition from the planar texture to the focal conic texture is designated as Vpf, 90 and Vpf, 10 before and after the transition state, respectively, and the threshold voltage of texture transition from the focal conic texture to the homeotropic texture is designated as Vfh, 10 and Vfh, 90 before and after the transition state, respectively.
Particularly, in the PNLC (polymer network liquid crystal) structure or the PDLC (polymer dispersed liquid crystal) structure, in which a polymer is added to a cholesteric liquid crystal, the bistability of a planar texture and a focal conic texture under no electric field is increased with interference at an interface between the cholesteric liquid crystal and the polymer (anchoring effect), whereby the state immediately after applying a pulse signal can be maintained for a long period of time.
In the stacked light modulating device using the technique, (A) the selective reflection state with a planar texture and (B) the transmission state with a focal conic texture are switched independently from the respective layers with only one switching signal, so as to realize color display having a memory effect under no electric field.
In the stacked light modulating device 101 subjected to image writing by the threshold value shifting method, the operation threshold values of the cholesteric liquid crystals of the display layers 107a and 107b are differentiated from each other, whereby arbitrary one or both of the display layers are in a reflection state, or both of them are in a transmission state, depending on a voltage applied between the transparent electrodes 105 and 106.
FIG. 16 is a graph showing a switching behavior of the cholesteric liquid crystals of the display layers 107a and 107b. As shown in FIG. 16, both the cholesteric liquid crystals of the display layers 107a and 107b are changed from a selective reflection state of a planar texture or a transmission state of a focal conic texture to a transmission state of a focal conic texture when the voltage externally applied with an electric power source 117 is increased, and changed from the focal conic texture to a homeotropic texture when the voltage is further increased, and both the liquid crystals are changed to a planar texture when the applied voltage is released.
However, the threshold values (voltages), on which the texture changes occur, are different from each other between the display layer 107a and the display layer 107b, as shown in FIG. 16. That is, the threshold voltage of texture change from a planar texture to a focal conic texture (which is hereinafter referred to as a lower threshold value) is Vpfa for the display layer 107a and Vpfb for the display layer 107b, which is higher than Vpfa of the cholesteric liquid crystal of the display layer 107a. The threshold voltage of texture change from a focal conic texture to a homeotropic texture (which is hereinafter referred to as an upper threshold value) is Vfpa for the display layer 107a and Vfpb for the display layer 107b, which is higher than Vfpa of the cholesteric liquid crystal of the display layer 107a. 
In the threshold value shifting method, the display layer 107a and the display layer 107b are independently controlled by utilizing the difference in threshold value.
More specifically, the display layers are selectively applied with a voltage within a range Vc, which is lower than the upper threshold value Vfpb of the display layer 107b but higher than the upper threshold value Vfpa of the display layer 107a, or a voltage within a range Vd, which is higher than the threshold value Vfpb of the display layer 107b, with the electric power source 117. In a part applied with the voltage with the range Vc, the display layer 107b is in a focal conic texture, and the display layer 107a is in a homeotropic texture. In a part applied with the voltage within the range Vd, the display layer 107a is in a homeotropic texture as similar to the part applied with the voltage within the range Vc, but the display layer 107b is changed to a homeotropic texture since the voltage exceeds the upper threshold value Vpfb.
Accordingly, the texture of the display layer 107b can be selected from either a focal conic texture or a homeotropic texture by selecting a voltage applied with the electric power source 117 from either a voltage within the range Vc or a voltage within the range Vd. When the applied voltage is quickly released in this stage, the homeotropic texture is changed to a planar texture, and the focal conic texture is maintained as it is. On the other hand, the display layer 107a is in a homeotropic texture with both the voltages applied before releasing, and the entire display layer 107a is changed to a planar texture by quickly releasing the voltage.
Thereafter, the display layers are selectively applied with a voltage within a range Va, which is lower than the lower threshold value Vpfa of the display layer 107a, or a voltage within a range Vb, which is higher than the lower threshold value Vpfa of the display layer 107a but is lower than the lower threshold value Vfpb of the display layer 107b, with the electric power source 117. In a part applied with the voltage with the range Vb, the display layer 107a undergoes texture change to a focal conic texture since the voltage exceeds the lower threshold value Vpfa, and in a part applied with the voltage within the range Va, the display layer 107a maintains the planar texture state since the voltage does not exceeds the lower threshold value Vpfa. On the other hand, the display layer 107b maintains the planar texture or the focal conic texture, which have been selected with respect to the upper threshold values, since both the applied voltages are lower than the lower threshold value Vfpb.
Consequently, the textures are selected with respect to each part of the display layer 107a to complete the writing operation.
That is, arbitrary one or both of the display layers 107a and 107b can be in a reflection state, or both of them can be in a transmission state, whereby a reflective image is displayed on the display surface. Accordingly, the two display layers can be independently controlled with one driving signal, which simplifies the structure of the medium and reduces the production cost.
There has been such a method as one embodiment of the driving method with the threshold value shifting method that a stacked light modulating device containing a photoconductor layer is used to enable switching by writing with exposure of light. FIG. 17 is a schematic cross sectional view showing a state of image writing by the threshold value shifting method using a stacked light modulating device containing a photoconductor layer. A stacked light modulating device 111 shown in FIG. 17 has two display layers 107a and 107b stacked between a pair of transparent electrodes 105 and 106 as similar to the stacked light modulating device 101 shown in FIG. 13, and further has a photoconductor layer 110 stacked between the display layer 107b and the transparent electrode 106. A colored layer 109 is further disposed between the photoconductor layer 110 and the display layer 107b. 
In this embodiment, the entire display layers are applied with a voltage (reset voltage) within a range Vc, which is lower than the upper threshold value Vfpb of the display layer 107b but higher than the upper threshold value Vfpa of the display layer 107a, as a bias voltage with the electric power source 117, and simultaneously, the display layers are selectively exposed, whereby the resistance of the photoconductor layer 110 of the exposed part is changed (lowered) to increase the partial voltage of the display layers 107a and 107b. According to the operation, the voltage applied to the display layers 107a and 107b in the exposed part exceeds the upper threshold value Vpfb. In other words, the exposed part is applied with a voltage within a range Vd, but the non-exposed part is still applied with the voltage within the range Vc. Accordingly, the same selection of texture change of the liquid crystal as in the selective switching operation with intensity of the voltage described with reference to FIG. 13 can be attained by on/off of exposure.
The aforementioned principle of the switching operation can also be applied to the lower threshold values. That is, the entire display layers are applied with a bias voltage within a range Va, which is lower than the lower threshold value Vpfa of the display layer 107a, with the electric power source 117, and simultaneously, the display layers are selectively exposed to select texture change in a part exceeding the lower threshold value Vpfa and a part not exceeding the lower threshold value Vpfa.
Consequently, upon applying a driving method using the threshold value shifting method to a stacked light modulating device containing a photoconductor layer, a voltage determined with respect to the upper threshold values and the lower threshold values is applied to the display layers, and while maintaining the voltage application, the display layers are selectively exposed to accomplish the writing operation.
In order to enhance the operation margin for driving the plural display layers independently by using the threshold value shifting method (which is, in short, the distances in voltage of the upper and lower threshold values between the display layers, i.e., the widths of the ranges Vb and Vc in FIG. 16), it is generally considered to increase the ratio of the operation threshold value and the electric field between the display layers, or to increase the ratio of the electric fields applied to the display layers, which are determined by the dielectric constant and the thickness of the display layer.
However, a cholesteric liquid crystal generally has relationships between the relative dielectric constant and the refractive index anisotropy Δn and between the refractive index anisotropy Δn and the reflectance, and therefore, in the case where the ratio in dielectric resistance between the display layers is increased, it is difficult to obtain sufficient brightness with the display layer having a lower dielectric constant. Furthermore, a cholesteric liquid crystal having such a tendency that the threshold electric field is decreased with increase of the specific dielectric constant, and therefore, the voltages applied to the display layers and the threshold electric fields of the display layers may be reversed to each other, which makes enhancement of the operation margin difficult.
Accordingly, the invention is to provide such a driving method and a driving device of a stacked light modulating device utilizing the threshold value shifting method that enhance the operation margin while maintaining sufficient reflectance in display layers of the stacked light modulating device, so as to realize stable threshold value shifting operation.